Light emitting device and drive method thereof

ABSTRACT

The reliability of an EL element is enhanced while the increase of the electric power consumption is suppressed. It becomes possible that in a SES drive, the reverse bias is applied to the EL element driven at a constant electric current. Moreover, the application of the reverse bias is performed by varying only the counter electrode, and thus withstand voltage of TFT and the increase of the electric power consumption due to the increase of voltage of the gate signal line drive circuit, which becomes a problem when changing greatly the electric current supplying line, can be suppressed. Furthermore, the reduction of the electric power consumption can also be achieved while the enhancement of the reliability is secured by making the reverse bias smaller than the forward bias. Moreover, the increase of the number of electric sources can be also suppressed by making the potential be in common with the potential of the electric source of the source signal line drive circuit or the gate signal line drive circuit, at the time when the reverse bias is applied.

This application is a continuation of U.S. application Ser. No.11/321,992, filed on Dec. 29, 2005 now U.S. Pat. No. 7,276,856 which isa divisional of U.S. application Ser. No. 11/147,527 filed on Jun. 8,2005 which is a continuation of U.S. application Ser. No. 10/376,366filed on Feb. 27, 2003 (now U.S. Pat. No. 7,023,141 issued Apr. 4,2006).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting element and a methodfor driving a light emitting device manufactured by forming thin-filmtransistors (hereinafter abbreviated as TFTs) on a substrate. Further,the present invention relates to electronic apparatuses using thelight-emitting device as a display unit.

Within this specification, as a representative example of the lightemitting element, an Electro Luminescence (EL) element will be used.Further, the EL element includes the ones which utilize emission oflight from singlet excitons (fluorescence) and the ones which utilizethe emission of light from triplet excitons (phosphorescence).

2. Description of the Related Art

In recent years, light-emitting devices having EL elements have beenvigorously developed as self light emitting elements. Unlike the liquidcrystal display devices, the light-emitting device is of self lightemitting type. The EL element has a structure in which an EL layer isheld between a pair of electrodes (anode and cathode), the EL layerbeing, usually, of a laminated-layer structure. Typically, there can beexemplified a laminated-layer structure of “positive hole-transportinglayer/light-emitting layer/electron-transporting layer”. This structurefeatures a very high light-emitting efficiency, and the EL displaydevices that have now been studied and developed have almost all beenemploying this structure.

There can be further exemplified a structure in which a positivehole-injecting layer, a positive hole-transporting layer, alight-emitting layer and an electron-transporting layer are laminated inthis order on the anode, or a structure in which the positivehole-injecting layer, the positive hole-transporting layer, thelight-emitting layer, the electron-transporting layer and theelectron-injecting layer are laminated thereon this order. Thelight-emitting layer may further be doped with a fluorescent pigment orthe like pigment.

In this specification, the layers provided between the cathode and theanode are all referred generally as an EL layer. Therefore, the abovepositive hole-injecting layer, positive hole-transporting layer,light-emitting layer, electron-transporting layer and electron-injectionlayer are all included in the EL layer.

A predetermined voltage is applied across the pair of electrodes (bothelectrodes) holding the EL layer of the above structure therein, wherebythe carriers are recombined in the light-emitting layer to thereby emitlight. At this time, the luminance of the EL element is in proportion toa current flowing to the EL element.

The light-emitting devices can include those of the passive matrix typeand those of the active matrix type. Here, the devices of the activematrix type are suited for the applications where a high-speed operationis required for the increase in the number of pixels accompanying highresolution and displaying moving images.

As a method of driving an EL element, there are a method of driving at aconstant voltage, in which a certain voltage is applied to the ELelement and a method of driving at a constant current, in which acertain electric current is flowed to the EL element. In the method ofdriving at a constant voltage, the electric resistance of an EL elementchanges depending upon the temperature variation and the amount ofelectric current flowing to the EL element also changes. Moreover, theelectric resistance of the EL element increases, and the amount ofelectric current flowing to the EL element decreases, due to the changesover time. Since the brightness of the EL element is in proportion tothe electric current, the brightness also changes along with those ofthe electric current. Hence, as a method of driving an EL element, itmay be desirable to employ the method of driving at a constant electriccurrent.

However, besides the change of electric resistance of the EL element,such deterioration occurs as the brightness of the EL element beinglowered due to the changes over time, even if a certain electric currentis flowed. Particularly, the lowering of the brightness due to thedeterioration of the EL element at the time of initial lighting, whichis called an “initial deterioration,” is significant.

Hence, for the purpose of suppressing the deterioration of the ELelement and enhancing the reliability, there is a method of applying thereverse bias to the EL element.

As for a reverse bias application voltage, it has been disclosed inJP-A-08-180972 gazette, that preferably the reverse bias voltage is madelarger than or equal to the forward bias voltage.

Moreover, as for a method of driving an active matrix type lightemitting device, there is a digital time gradation method which is noteasily influenced by the variation of the characteristics of the TFT fordriving. That is a method in which each pixel is configured with twotransistors, a TFT for driving and a TFT for switching, one frame periodis divided into an address (writing) period and a sustain (lightemitting) period, and the gradation is controlled by the sum of thelengths of the time for emitting the light.

Moreover, there is a digital time gradation method in whichpoly-gradation display with a high precision can be realized byutilizing three transistors, specifically, a TFT for driving, a TFT forswitching and a TFT for blanking. In the present specification, adigital time gradation method using these three transistors for eachpixel is defined as SES (Simultaneous Erase Scan) drive. It should benoted that concerning with this SES drive, the detail of it has beendisclosed in JP-A-2001-343933 gazette.

It is contemplated that the reverse bias is applied by the SES drive atthe time when an EL element is driven at a constant electric current. Inan active matrix type light emitting device, in order to drive the ELelement at the constant electric current, the TFT for driving and the ELelement are put in series and in order to operate the TFT for driving inthe saturation region, a high voltage is required.

Furthermore, in the case where the reverse bias voltage is made largerthan the forward bias voltage, if an electrode in which the capacitanceloading is large is changed, such a problem occurs as increase of theconsumption of the electric power. Moreover, it is considered thatmalfunctions indicated in the followings may occur depending upon thereverse bias application method. The pixel configuration in the casewhere the foregoing SES drive method is performed is shown in FIG. 2.Moreover, the reason will be described with reference to FIG. 2.

Each pixel has a source signal line 201, a gate signal line for writing202, a gate signal line for blanking 203, a TFT for switching 204, a TFTfor blanking 205, a TFT for driving 206, an EL element 207 and anelectric current supplying line 208, one of EL elements 207 is connectedto a source electrode of the TFT for driving 206 or a drain electrode(pixel electrode), and the other is connected to a counter electrode209.

Here, the TFT for switching 204 and the TFT for blanking 205 areN-channel type TFTs, the TFT for driving 206 is a P-channel type TFT,and in the EL element 207, the side connected to the TFT for driving 206is made an anode, the side connected to the counter electrode 209 ismade a cathode. For the purpose of clarifying the description, eachpotential is shown within the parenthesis ( ) as an example. However,these potentials are solely examples, when it is driven by the followingmethods, it may be appropriately set at the desired potential.

First, a pulse (8V) by which the TFT for switching 204 is turned ON isinputted into the gate signal line for writing 202, the TFT forswitching 204 is turned ON, and a picture signal outputted into thesource signal line 201 is applied to a gate electrode of the TFT fordriving 206. Here, since the TFT for driving 206 is a P-channel typeTFT, when the picture signal is at H level (6 V), it is turned OFF, andwhen it is at L level (0 V), it is turned ON.

Subsequently, by turning the TFT for driving 207 ON, the electriccurrent flows through the EL element 207 from the electric currentsupplying line 208 (5 V) towards the counter electrode 209, and thelight emits. The TFT for driving 206 is operated in the saturationregion. Moreover, when the TFT for driving 206 is turned OFF, theelectric current does not flow into the EL element 207, it becomes in anon-light emitting state.

Subsequently, a pulse (8V) for turning the TFT for blanking ON isinputted into the gate signal line for blanking 203, and the TFT forblanking 205 is turned ON. The potential of the electric currentsupplying line 208 is inputted to the gate electrode of the TFT fordriving 206 by turning the TFT for blanking 205 ON, the voltage betweenthe gate and the source of the TFT for driving 206 becomes 0, the TFTfor driving 206 is turned OFF. Therefore, the EL element 207 becomes ina non-light emitting state.

Here, the reverse bias period is provided in a non-light emittingperiod, and consider the case where the reverse bias is applied to theEL element 207. In the case where the reverse bias is applied by greatlychanging the potential of the electric current supplying line 208, thepotential at the time when the gate signal line for writing 202 isturned OFF must be increased.

For example, when the potential of the electric current supplying line208 was greatly changed (5 V→−22 V), the TFT for blanking 205 is turnedON and the potential of the gate electrode of the TFT for driving 206becomes the potential of the electric current supplying line 208 (−22 V)since the potential of the gate electrode of the TFT for blanking 205(−2 V) is higher than the potential of the electric current supplyingline 208 by the portion of more than the threshold of the TFT forblanking 205. Therefore, the TFT for driving 206 is also turned ON, thepotential of the pixel electrode becomes the potential raised from thepotential of the gate electrode of the TFT for driving 206 by theportion of the threshold voltage of the TFT for driving 206 (about −20V). Therefore, as a result, the reverse bias voltage (about 10 V) isapplied to the EL element.

However, when paying attention to the TFT for switching 204 at thistime, the TFT for switching 204 is turned ON since the potential of thegate electrode (−2 V) is higher than the potential of the gate electrodeof the TFT for driving 206 (about −20 V) by the portion of more than thethreshold of the TFT for switching 204. Specifically, the electriccurrent supplying line 208 and the source signal line 201 are shortedwhile sandwiching the TFT for switching 204, the TFT for blanking 205.In this way, in order to prevent the TFT which originally should not beturned ON from being turned ON, the potential at the time when the gatesignal line for writing 202 must be further lowered (about −24 V).However, in this case, the increase of the consumption of electric powerof the gate driver becomes a problem by increasing the voltage amplitudeof the signal as well as the uncertainty is generated on the withstandvoltage of the TFT.

Moreover, by changing greatly the reverse bias voltage, the othersection which has capacitive coupling to the section for changing thevoltage (electric current supplying line 208 and the like) is changed bythe voltage, being influenced at the time when the reverse bias isapplied. Due to this, it is also considered that the transistor whichshould be turned OFF is turned ON, or the consumption of electric powerare increased by charging and discharging the moved electric charge andso forth.

Moreover, by increasing the reverse bias voltage (10V) more than theforward bias voltage (8V), the changes of the potential of the electriccurrent supplying line 208 (27 V) further become larger, even if thedeterioration of the EL element is suppressed, the demerit such as theincrease of the consumption of electric power and the like cannot beavoided.

Hence, the present invention aims at suppressing the increase of theconsumption of electric power and enhancing the reliability of the ELelement, and proposes an alternate current drive method in which thereverse bias is applied to the EL element driving at the constantelectric current in the SES drive.

SUMMARY OF THE INVENTION

As for a method of driving an EL element at a constant electric currentin the SES drive, one example has been described in Japanese PatentApplication No. 2002-025065. In this SES drive, the reverse biasapplication period is provided in a non-light emitting period.

As a method of applying the reverse bias, it is characterized in thatthe electrode on the opposite side of the electrode (counter electrode)directly connected to the drain electrode or the source electrode of theTFT for driving while sandwiching the EL element is changed.

Furthermore, the consumption of electric power is reduced by making thereverse bias voltage smaller than the forward bias voltage. Even at thistime, the effect of enhancing the reliability by utilizing the reversebias is obtained.

Moreover, the increase of the number of electric sources within thepanel can be prevented by making the electric source at the time whenthe reverse bias is applied to the counter electrode be in common withthe driver electric source within the panel. Here, the forward biasvoltage is gradually increased from the light emitting initiationvoltage in order to drive the EL element at the constant electriccurrent.

Hereinafter, the configuration of the present invention will bedescribed.

A method of driving a light emitting device of the present invention is,

a method of driving a light emitting device having a plurality of pixelsin which light emitting elements are provided and performing theexpression of the gradation by controlling the difference between thelight emitting times of the foregoing light emitting elements, theforegoing method is characterized in that it comprises,

a first step of performing the writing to the foregoing pixel of apicture signal,

a second step of making the foregoing light emitting element in a lightemitting state or in a non-light emitting state based on the foregoingwritten picture signal,

a third step of forcibly making the foregoing light emitting elementinto a non-light emitting state after the foregoing second step, and

a fourth step of applying the reverse bias voltage V₂ whose polarity isinversed between the first electrode and the second electrode of theforegoing light emitting element with respect to the forward biasvoltage V₁ applied between the first electrode and the second electrodeof the foregoing light emitting element at the time when the foregoinglight emitting element emits, and

the device is made so as to hold the expression of |V₁|≧|V₂|.

A method of driving a light emitting device of the present invention is,

a method of driving a light emitting device having a plurality of pixelsin which light emitting elements are provided and performing theexpression of the gradation by controlling the difference between thelight emitting times of the foregoing light emitting elements, theforegoing method is characterized in that,

one frame period has n pieces (n is a natural number, 2<n) of subframeperiods, the foregoing subframe period has an address period in whichthe picture signal is written into the pixel and a sustain period inwhich the display is performed by controlling the light emitting ornon-light emitting of the foregoing light emitting element based on thepicture signal written in the foregoing pixel, respectively,

m pieces (m is a natural number, 0<m≦n−1) of subframe periods selectedfrom n pieces of the foregoing subframe periods have m pieces of resetperiods in which the writing of the reset signal is performed into thepixel after the foregoing sustain period has been completed and do notoverlap with each other, and m pieces of blanking periods in which thestate of the foregoing light emitting element is forcibly made into anon-light emitting state in a line in which a reset signal is writtenand the periods do not overlap each other, respectively,

k pieces (k is a natural number, 0<k≦m) of subframe periods selectedfrom the foregoing m pieces of subframe periods have k pieces of thereverse bias application periods in which at the time when the lightemitting element emits, the reverse bias voltage V₂ whose polarity isinversed is applied with respect to the forward bias voltage V₁ appliedbetween the first electrode and the second electrode of the foregoinglight emitting element and the periods do not overlap with each other,respectively,

The foregoing address period, the foregoing sustain period, theforegoing reset period, the foregoing blanking period, the foregoingreverse bias period and the foregoing reverse bias application periodhave a period in which one portion of these periods is overlapped witheach other, respectively,

And the device is made so as to hold the expression of |V₁|≧|V₂|.

In the above-described method of driving a light emitting device of thepresent invention, the foregoing method is characterized in that,

in a period in which the display of the screen is performed by theprocedure that the foregoing light emitting element emits or becomes ina non-light emitting state, the light emitting element to which theforegoing forward bias voltage is applied is driven at the constantelectric current.

In the above-described method of driving a light emitting device of thepresent invention, the foregoing method is characterized in that,

the changing from the foregoing forward bias voltage to the foregoingreverse bias voltage is performed by changing only the counter electrodeof the foregoing light emitting element.

BRIEF DESCRIPTION ON THE DRAWINGS

FIGS. 1A to 1E are diagrams for illustrating one Embodiment of thepresent invention;

FIG. 2 is a diagram showing a pixel configuration of a light emittingdevice configured using three transistors;

FIGS. 3A to 3D are graphical representations showing the reliabilitytest results of an EL element;

FIGS. 4A and 4B are block diagrams showing the configuration of a lightemitting device;

FIG. 5 is a diagram showing one example of configuration of a sourcesignal line drive circuit;

FIG. 6 is a diagram showing one example of configuration of a gatesignal line drive circuit;

FIGS. 7A and 7B are examples showing an element layout of a pixelsection and its cross section;

FIGS. 8A and 8B are diagrams showing an example of configuration of agate signal line drive circuit;

FIG. 9 is a drawing showing an example of a light emitting deviceself-contained in an electronic apparatus; and

FIGS. 10A to 10E are drawings showing examples of electronic apparatusesto which the present invention can be applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

A pixel configuration and its driving method for the purpose of solvingthe problem will be described with reference to FIG. 1D below.

The respective pixels have a source signal line 111, a gate signal linefor writing 112, a gate signal line for blanking 113, a TFT forswitching 115, a TFT for blanking 116, a TFT for driving 117, an ELelement 118 and an electric current supplying line 114, respectively,one side of the EL element is connected to a source electrode of the TFTfor driving 117 or a drain electrode (pixel electrode), the other sidehas been connected to the counter electrode 119. Moreover, a switch 120is connected to the counter electrode 119, at the time when the forwardbias is applied, it can be switched to the potential of Vc1 and at thetime when the reverse bias is applied, it can be switched to thepotential of Vc2.

Here, the TFT for switching 115, the TFT for blanking 116 are N-channeltype TFTs, the TFT for driving 117 is a P-channel type TFT, and in theEL element 118, the side connected to the TFT for driving 117 is made asan anode, the side connected to the counter electrode 119 is made as acathode. Moreover, for the purpose of making the description clearer,examples of voltage values are shown within the parenthesis ( ).However, these potentials are solely examples, when it is driven by thefollowing methods, it may be appropriately set at the desired potential.

According to the picture signal, in the period in which the forward biasis applied to the light emitting EL element 118, the potential of thecounter electrode 119 becomes the potential of Vc1 (−10 V) by the switch120. First, a pulse is inputted into the gate signal line for writing112, it becomes H level (8V), the TFT for switching 115 is turned ON,the picture signal outputted to the source signal line 111 is applied tothe gate electrode of the TFT for driving 117. Here, since the TFT fordriving 117 is a P-channel type TFT, at the time when the picture signalis at H level (6 V), it is turned OFF, and at the time when it is at Llevel (0 V), it is turned ON.

Subsequently, by turning the TFT for driving 117 ON, the electriccurrent flows through the EL element 118 from the electric currentsupplying line 114 (5 V) towards the counter electrode 119 whosepotential is at Vc1 (−10 V), and the light emits. The TFT for driving117 operates in the saturation region. Moreover, when the TFT fordriving 11 is turned OFF, the electric current does not flow into the ELelement 118, it becomes in a non-light emitting state.

Subsequently, a pulse is inputted into the gate signal line for blanking113, it becomes H level (8 V), and the TFT for blanking 116 is turnedON. The potential of the electric current supplying line 114 (5V) isinputted to the gate electrode of the TFT for driving 117 by turning ONthe TFT for blanking 116, the voltage between the gate and the source ofthe TFT for driving 117 becomes 0, the TFT for driving 117 is turnedOFF. Hence, the EL element 118 becomes in a non-light emitting state.

In the case where the reverse bias period is applied to the EL element118, the potential of the counter electrode 119 is switched to thepotential of Vc2 (10 V) by the switch 120. This potential may be made incommon with the potential of the source signal line drive circuit or thegate signal line drive circuit. Here, it is made in common with theelectric source of the gate signal line drive circuit. The capacitancebetween the counter electrode 119 (cathode) and the pixel electrode(anode) is large, the potential of the pixel electrode is raised to thedirection of plus by the change (20 V) at the time when the potential ofthe counter electrode 119 is switched from Vc1 to Vc2. At this time,since the potential of the gate electrode of the TFT for driving 117 ismaintained at the voltage (5 V) at the time of blanking, the potentialof the pixel electrode becomes a value (about 7 V) that the thresholdvoltage (about 2 V) of the TFT for driving 117 has been added to thepotential (5V) of the gate electrode of the TFT for driving 117, wherebythe reverse bias voltage (about 3 V) is applied to the EL element 118.

By the above-described operation, the reverse bias voltage is applied tothe EL element, thereby being capable of suppressing the initialdeterioration of the EL element which has been considered as a problem.Furthermore, at the time when the reverse bias voltage is applied, onlythe counter electrode is changed, in the TFT for driving and the TFT forblanking connected to the electric current supplying line, the voltagebetween the gate and the source is not influenced at all. Hence, thisdoes not increase significantly the consumption of electric power of thegate signal line drive circuit.

Embodiment 2

A pixel configuration and its driving method for the purpose of solvingthe problem will be described with reference to FIG. 1E below.

The respective gate signal lines for writing have a switch 121, at thetime when the forward bias is applied, Vc3 is selected and at the timewhen the reverse bias is applied, Vc4 is selected. At the time when Vc3was selected, an output pulse from a shift register of the gate driveris out putted to the gate signal line for writing, and at the time whenVc4 was selected, the potential at which the TFT for switching becomesin an ON state is outputted to the gate signal line for writing of alllines at the same time.

The operation at the time when the forward bias is applied is the samewith that of Embodiment 1. At the time when the reverse bias is applied,the potential of the counter electrode is switched to Vc2 (8V). Theelectric source of Vc2 may be made in common with the electric source ofthe source signal line drive circuit or the gate signal line drivecircuit. Here, it is made in common with the electric source of thesource signal line drive circuit

Next, the source signal line drive circuit is operated so that all ofthe potentials of the source signal line become L level (0 V).

Moreover, the potentials of the gate signal lines for writing of alllines are made Vc4 (10 V) by the switch 121, whereby the TFTs forswitching are all turned ON at the same time, the potential of the gateelectrode of the TFT for driving is the same with the potential of thesource signal line, that is, L level (0 V). Hence, the TFT for drivingis turned ON, the potential of the pixel electrode becomes the potentialof the electric current supplying line (5 V), and the reverse biasvoltage (3 V) is applied.

According to the present Embodiment, compared to Embodiment 1, thepotential of the pixel electrode at the time when the reverse bias isapplied is lowered by the portion of the threshold voltage of the TFTfor driving, whereby in the case where the same reverse bias voltage isapplied, the potential of the counter electrode at the time when thereverse bias is applied can be lowered by the portion of the thresholdvoltage of the TFT for driving, and the consumption of electric powercan be reduced.

EXAMPLES

Hereinafter, Examples of the present invention will be described.

Example 1

As shown in FIG. 9, in the case where the light emitting device is usedas a display part of an electronic apparatus of a cellular phone or thelike, it is self-contained in a form of a light emitting device 901.Here, the light emitting device 901 indicates the form in which a paneland a substrate on which a LSI for processing a signal for driving thelight emitting device, a memory and the like have been mounted areconnected with each other.

FIG. 4A shows a block diagram of the light emitting device 901. Thelight emitting device 901 has a panel 450 and a drive circuit 460.

The drive circuit 460 has a signal generation section 411 and anelectric source section 412. The electric source section 412 generateselectric sources having a plurality of the desired voltage values,respectively, from the electric source supplied by the external batteryand supplies to the source signal line drive circuit, the gate signalline drive circuit, a light emitting element, the signal generationsection 411 and the like, respectively. An electric source, a picturesignal and a synchronization signal are inputted into the signalgeneration section 411, a clock signal and the like for driving thesource signal line drive circuit and the gate signal line drive circuitare generated as well as the conversions of a variety of signals areperformed so as to be capable of processing in the display device 450.Moreover, a selection section 413 for switching the input electricsource between the drive circuit 460 and the panel 450 is provided.

Moreover, the panel 450 is configured with a pixel section 401, a sourcesignal line drive circuit 402, a gate signal line drive circuit forwriting 403, a gate signal line drive circuit for blanking 404, anelectric current supplying line 405, a counter electrode contact 406, aFPC 407 and the like on the substrate. At the center section of thesubstrate, the pixel section 401 is arranged, and in the peripheralsection, the source signal line drive circuit 402, the gate signal linedrive circuit for writing 403, the gate signal line drive circuit forblanking 404 and the like are arranged. The supply of the signal andelectric source for driving the source signal line drive circuit 402,the gate signal line drive circuit for writing 403, the gate signal linedrive circuit for blanking 404 is performed through the FPC 407 by thedrive circuit 460. The counter electrode of the EL element is formed onthe whole surface of the pixel section 401, the potential is giventhrough the FPC 407. The potential of this counter electrode can beswitched by the selection section 413 in order to apply the reversebias.

FIG. 4B is an example in which the selection section 413 is integrallyformed on the panel 450.

Moreover, the panel 450, the selection section 413, the signalgeneration section 411 and the electric source section 412 may be alsointegrally formed on the substrate.

Example 2

A schematic diagram of a source signal line drive circuit is shown inFIG. 5 and a schematic diagram of a gate signal line drive circuit isshown in FIG. 6 in the case where the display of the picture image isperformed using a digital picture signal.

The source signal line drive circuit has a shift register 502 using aplurality of D-flip flops 501, a first latch circuit 503 a, a secondlatch circuit 503 b, a level shifter 504, a buffer 505 and the like.Signals inputted from the external are a clock signal (S-CK), aninversed clock signal (S-CKb), a start pulse (S-SP) and a digitalpicture signal (Digital Video Data). In the case of the configurationlike FIG. 5, digital picture signals are inputted, for example, in sucha way as “in the first bit, first line→second line→ . . . →final line;in the second bit, first line→second line→ . . . →final line; in thethird bit, first line→second line . . . ” in series.

First, according to the timings of a clock signal, a clock invertedsignal and a start pulse, the sampling pulses are in turn outputted fromthe shift register 502. Subsequently, the sampling pulse is inputtedinto the first latch circuit 503 a, takes in and maintains the digitalpicture signals of the respective rows at the timing that the samplingpulses have been inputted.

When the maintaining of the digital picture signal is completed in thefirst latch circuit 503 a at the final stage, a latch pulse is inputtedduring the horizontal fly-back period, and in this timing, the digitalpicture signals maintained in the first latch circuit 503 a aretransmitted to the second latch circuit 503 b at the same time. Then, inthe level shifter, these are subjected to the conversion of theamplitude of the pulses, subsequently after the picture signal waveformhas been shaped in the buffer, these are outputted to source signallines S1-Sx.

On the other hand, the gate signal line drive circuit has a shiftregister 602 composed of a plurality of D-flip flops 601, a levelshifter 603, a buffer 604 and the like. Signals inputted from theexternal are a clock signal (G-CK), an inversed clock signal (G-CKb) anda start pulse (G-SP).

First, according to the timings of a clock signal, a clock inversedsignal, and a start pulse, pulses are in turn outputted from the shiftregister 602. Subsequently, these are subjected to the conversion of theamplitude of the pulse in the level shifter 603, and subsequently, afterthe pulse waveform has been shaped in the buffer, these are outputted tothe respective gate signal lines as pulses for in turn selecting gatesignal lines G1-Gy. When the selection on the final line Gy isterminated, after passing through the vertical fly-back period, again,the pulses are outputted from the shift register 602, and the selectionsof the gate signal lines are in turn performed.

Example 3

The actual driving timing in the case where the pixel configurationlisted in Embodiment 1 is SES driven and the reverse bias is appliedwill be described below with reference to FIG. 1. FIGS. 1A and 1B showtiming charts, FIG. 1 C shows potential of the counter electrode, andFIG. 1 D shows the pixel configuration.

As shown in FIG. 1 A, one frame period is divided into four subframeperiods SF1-SF4. The respective subframes have an address (writing)period Ta and a sustain (light emitting) period Ts. Moreover, in asubframe in which an address (writing) period Ta is longer than asustain (light emitting) period Ts (where SF3 and SF4 are relevant), ablanking period Tr, a non-light emitting period Te and a reverse biasperiod Tb as well as the address (writing) period Ta and the sustain(light emitting) period Ts are provided.

The address (writing) period Ta is a period in which a digital picturesignal is written in the pixel, the sustain (light emitting) period Tsis a period in which the display is performed by making the EL elementin a light emitting state or in a non-light emitting state based on thedigital picture signal written in the address (writing) period Ta. Thelight emitting time of each pixel per one frame period is determineddepending upon the fact that in which subframe the EL element emits thelight, and the gradation display is performed using the differencebetween these light emitting times.

The blanking period Tr is a period in which a signal for forcibly makingthe EL element into a non-light emitting state is inputted, thenon-light emitting period Te is a period in which the EL element becomesin a non-light emitting state based on the signal inputted in theblanking period Tr. Moreover, the reverse bias period Tb is a period inwhich the reverse bias is applied to the EL element.

As for an operation, first, in an address (writing) period Ta1 of SF1,the pulses are in turn inputted into the gate signal line for writing112 from the first line, a digital picture signal for the purpose ofturning the TFT for switching 115 ON, turning the TFT for driving 117 ONand OFF is written in each pixel. On a line where the digital picturesignal is written, it is immediately transferred to the sustain (lightemitting) period Ts1. When the writing work is completed from the firstline to the final line, the address (writing) period Ta1 is terminated.After the sustain (light emitting) period Ta1 has been terminated,again, the pulses are in turn inputted into the gate signal line forwriting 112 from the first line, the address (writing) period Ta2 of SF2is initiated, and transferred to the sustain (light emitting) periodTs2.

Subsequently, the address (writing) period Ta3 of SF3 is initiated.Here, since the sustain (light emitting) period Ts3 is shorter than theaddress (writing) period Ta3, immediately after the sustain (lightemitting) period Ts3 has been terminated, it cannot transfer to the nextaddress (writing) period Ta4. Therefore, when the sustain (lightemitting) period Ts3 of the respective lines is terminated, the blankingperiod Tr3 is initiated. In the blanking period Tr3, the pulses are inturn inputted from the first line to the gate signal line for blanking113, the TFT for blanking 116 is turned ON, the light emitting of the ELelement 118 is forcibly stopped, and thereinafter, it becomes anon-light emitting period Te3.

At this time, in the usual SES drive method, the length of a non-lightemitting period is a period from the time when the sustain (lightemitting) period of the first line is terminated to the time when theaddress (writing) period of the final line is terminated. In a non-lightemitting period Te, since the potential of the gate electrode of the TFTfor driving 117 is maintained, the TFT for driving 117 continues to bein an OFF state.

After the non-display period Te3 has been terminated, it is transmittedto the reverse bias period Tb3. Since in the reverse bias period Tb3,the potential of the counter electrode is changed as shown in FIG. 1 C,after the blanking period Tr3 is terminated, the non-light emittingperiod Te3 is extended more than that at the time when the usual SESdrive is performed, which is necessary to provide it during theforegoing non-light emitting period.

In the reverse bias period Tb3, the potential of the counter electrodeis heightened more than the potential of the pixel electrode and thereverse bias is applied to the EL element 118 by switching the switch120 from the Vc1 to Vc2.

At the time when the desired reverse bias period Tb3 has beenterminated, the potential of the counter electrode is switched from Vc1to Vc2. At the same time, the next address (writing) period Ta4 isinitiated. Hereinafter, the similar operation as SF3 is repeated, whenthe SF4 is terminated, one frame is terminated, and transferred to thenext frame.

Moreover, in FIG. 1D, a condenser is not shown between the gateelectrode of the TFT for driving 117 and the electric current supplyingline 114, but a condenser which maintains the potential of the gateelectrode of the TFT for driving 117 for a certain time may be provided.Moreover, if the channel capacitance of the TFT for driving 117 issufficient, it is not necessary to provide the condenser. Moreover, theconfiguration of the gate signal line drive circuit for blanking ischanged, and in a non-light emitting period, it may be made as a voltagebetween the gate and the source so that the TFT for driving 117 issecurely turned OFF.

Moreover, here, although only in the subframe in which the address(writing) period Ta is longer than the sustain (light emitting) periodTs, a blanking period Tr, a non-light emitting period Te and a reversebias period Tb are provided, also in the subframe in which the address(writing) period Ta is shorter than or equal to the sustain (lightemitting) period Ts, the respective periods are provided, the reversebias may be applied to the EL element.

Moreover, in FIG. 1A, after the blanking period Tr has been completelyterminated until the final line, the reverse bias period Tb wasinitiated, but as shown in FIG. 1 B, the final line of the blankingperiod Te and the initiation of the reverse bias application period Tb3are overlapped with each other, and a better duty rate can also beobtained.

Example 4

The gate signal line drive circuit for writing will be described withreference to FIG. 8A and the pixel configuration will be described withreference to FIG. 8B in the case where the pixel configuration listed inEmbodiment 2 is SES driven and the reverse bias is applied.

The gate signal line drive circuit of FIG. 8A has a shift register 802composed of a plurality of stages of D-flip flops 801, a level shifter803, a buffer 804, a selection circuit 805 and the like. Signalsinputted from the external are a clock signal (G-CK), an inversed clocksignal (G-CKb), a start pulse (G-SP), and a select signal (G-SEL). Aswitch 821 shown in FIG. 8A corresponds to the selection circuit 805 inFIG. 8B.

First, in the forward bias period, according to the timings of a clocksignal, a clock inversed signal and a start pulse, the pulses are inturn outputted from the shift register 802. Subsequently, it issubjected to the conversion of amplitude of the pulse by the levelshifter 803, subsequently, after a pulse waveform has been shaped in thebuffer, it is inputted into the selection circuit 805. In the selectioncircuit 805, the output pulses from the buffer 804, which have beenselected by a select signal are in turn outputted to the gate signallines for writing G1-Gy. When the selection is terminated on the finalline Gy, after these have passed through the vertical fly-back period,pulses are again outputted from the shift register 802, and the gatesignal lines are in turn selected.

Moreover, in the reverse bias period, a select signal is switched, andregardless of the buffer output, the potential at which the TFT forswitching 815 is turned ON is outputted to the gate signal lines forwriting G1-Gy at the same time. At this time, the potential of L levelhas been inputted into all of the rows of source signal lines 811.Hence, the TFT for switching 815 of all the pixels are turned ON at thesame time, the potential of L level is inputted to the gate electrode ofthe TFT for driving 817, and the TFT for driving 817 is turned ON. Byturning the TFT for driving 817 ON, the potential of the electriccurrent supplying line 814 is inputted to the pixel electrode.

In the reverse bias period, since the potential of the counter electrode819 is switched from Vc1 to Vc2, the potential of the counter electrode819 becomes higher than the potential of the pixel electrode and thereverse bias is applied to the EL element.

Example 5

FIG. 7A shows that an example of element layout in the case that pixelshaving structures of FIGS. 1D, 8B shown in Embodiments 1, 2 are actuallyformed. Further, FIG. 7B shows a cross-sectional view of FIG. 7A takenalong the line X-X′.

In FIG. 7A, reference numeral 700 is a substrate having an insulatingsurface. A driving TFT 707 is formed on a substrate 700. Then, sourceand drain electrodes made from wiring materials are formed to connectwith impurity regions, which form source and drain regions of thedriving TFT 707. Either is formed to connect with a pixel electrode 708on the overlapping portions. An organic conductive film 712 is formed onthe pixel electrode 708, and an organic thin film (light emitting layer)713 is formed thereon. An opposite electrode 714 is formed on theorganic thin film (light emitting layer) 713. The opposite electrode 714is formed all over the surface so as to connect uniformly with allpixels.

In this specification, reference as “EL element” means a laminated layerincluding the pixel electrode 708, the organic conductive film 712, theorganic thin film (light emitting layer) 713, and the opposite electrode714 in FIG. 7B. Either the pixel electrode 708 and the oppositeelectrode 714 is an anode, or the other is a cathode.

Light generated at the light thin film (light emitting layer) 713radiates through either the pixel electrode 708 or the oppositeelectrode 714. In FIG. 7B, when light is emitted downward through thepixel electrode side or the side having TFT and the like, it is referredto as a bottom emission structure. When light is emitted upward throughthe opposite electrode side, it is referred to as a top emissionstructure.

In the case of the bottom emission structure, the pixel electrode 708 ismade from a transparent conductive film. In the case of the top emissionstructure, the opposite electrode 714 is made from a transparent film.

The structure shown in this example is illustrative only. A laminationorder of a pixel layout, a cross-sectional structure, and the electrodeof EL elements are not to be considered limited to this structure.

In the light emitting element for color display, EL elements for R, G, Bcolor may be deposited separately. Otherwise, monochromatic EL elementsmay be formed so that the structure achieves R, G, B color by colorfilters.

Example 6

In the present Example, a macromolecular compound is applied as a lightemitting layer, and further, in a light emitting element in which abuffer layer consisting of an electrically conductive macromolecularcompound is provided between the anode and the light emitting layer, theresults that the measurement on the brightness deterioration at the timewhen the direct current drive (forward bias is always applied) and thealternate current drive (forward bias and reverse bias are alternatelyapplied in a certain cycle) will be described below.

FIGS. 3A and 3B show the results of the reliability test when thealternate drive has been carried out at the forward bias; 3.7 V, thereverse bias; 1.7 V, duty ratio; 50% and alternate current frequency; 60Hz. FIG. 3A is a graphical representation using a linear scale, and FIG.3B is a graphical representation using a LOG scale. The initialbrightness has been about 400 cd/cm². For the purpose of comparing, theresults of the reliability test at the time when the direct currentdrive (forward bias; 3.65 V) has been carried out are also shown at thesame time. As a result, in the direct current drive, the brightness hasbeen reduced by half at about 400 hours, in contrast to this, in thealternate current drive, even after about 700 hours have passed, theinitial brightness have not been reduced by half.

FIGS. 3C and 3D show the results of the reliability test at the timewhen the alternate current drive has been carried out at the forwardbias; 3.8 V, the reverse bias; 1.7 V, duty ratio; 50% and alternatecurrent frequency; 600 Hz. FIG. 3C is a graphical representation using alinear scale, and FIG. 3D is a graphical representation using a LOGscale. The initial brightness has been about 300 cd/cm². For the purposeof comparing, the results of the reliability test at the time when thedirect current drive (forward bias; 3.65 V) has been carried out arealso shown at the same time. As a result, in the direct current drive,the brightness has been reduced by half at about 500 hours, in contrastto this, in the alternate current drive, about 60% of the initialbrightness was maintained even after about 700 hours have passed.

Example 7

Since the light emitting device using a light emitting element is aself-luminous type light emitting device, when comparing to a liquiddisplay, the visibility is more excellent at the bright place, and thevisual field is wider. Therefore, it can be used for the display part ofa variety of electronic apparatuses.

As an electronic apparatus in which a light emitting device of thepresents invention is used, a television, a video camera, a digitalcamera, a goggle type display (head mount display), a navigation system,an audio system (car audio, audio component and the like), a notebooksized personal computer, a game machine, a handheld terminal (mobilecomputer, cellular phone, portable type game machine, electronic book orthe like), an image reproduction device equipped with a recording medium(device equipped with a display capable of reproducing the recordingmedium, concretely, such as an image Digital Versatile Disc (DVD)) andthe like are listed. Particularly, in the case of handheld terminals inwhich the occasions of seeing the screen from the slanting angle arefrequent, since the width of the visual field is considered to beimportant, it is desirable to use a light emitting device. Concreteexamples of these electric devices are shown in FIG. 10.

FIG. 10A is an EL display, including a case 1001, an audio output part1002, a display part 1003 and the like. A light emitting device of thepresent invention can be used in the display part 1003. Since the lightemitting device is a self-luminous type light emitting device, the backlight is not required, whose display part can be made thinner than aliquid display. It should be noted that the light emitting elementdisplay device includes all kinds of display devices for informationdisplay such as a display for personal computer, a display for receivingTV broadcast, a display for advertising and the like.

Moreover, FIG. 10C shows a large EL display, and includes a case 1021,an audio output part 1022 and a display part 1023, as similar to FIG.1A. A light emitting device of the present invention can be used in thedisplay part 1023.

FIG. 10B shows a mobile computer, including a main body 1011, a stylus1012, a display part 1013, an operation bottom 1014, an externalinterface 1015 and the like. A light emitting device of the presentinvention can be used in the display 1013.

FIG. 10D shows a game machine, including a main body 1031, a displaypart 1032, an operation bottom 1033 and the like. A light emittingdevice of the present invention can be used in the display part 1032.

FIG. 10E shows a cellular phone, including a main body 1041, an audiooutput part 1042, an audio input part 1043, a display part 1044, anoperation switch 1045, an antenna 1046 and the like. A light emittingdevice of the present invention can be used in the display part 1044. Itshould be noted that the display part 1044 could suppress theconsumption of electric current of the cellular phone by displaying awhite letter on the black background.

It should be noted that when the luminous brightness of an organic lightemitting material will be enhanced in the future, the light containingoutputted image information is enlarged and projected using a lens orthe like and is capable of being used for a projector of front type orrear type.

Moreover, the above-described electronic apparatuses frequently havedisplayed information delivered through an electronic communication linesuch as the internet, CATV (cable TV) and the like, particularly, theoccasions for displaying a dynamic picture information has beenincreased. Since response rate of an organic luminescent material isvery high, it is preferable to use a light emitting device for dynamicdisplay.

Moreover, since the colors are brilliant, it is also suitable for arealistic large screen television.

Moreover, since as for a light emitting device, the portion where thelight emits consumes the electric power, it is desirable to displayinformation in the way that the light emitting portion is reduced to thesmall portion. Therefore, in the case where a light emitting device isused in the display part in which information using characters is mainlyused such as a handheld terminal, particularly, a cellular phone, anaudio system, it is desirable to drive so that information usingcharacters is formed by the light emitting portion using the non-lightemitting portion as its background.

As described above, the application range of the present invention isextremely wide, the present invention is capable of being used forelectronic apparatuses in any field. Moreover, as for an electronicapparatus of the present Example, light emitting devices of anyconfiguration shown in Examples 1-5 may be used.

According to a light emitting device of the present invention, in a SESdrive method, the reliability can be enhanced by applying the reversebias to an EL element driven at the constant electric current. Moreover,the application of the reverse bias is performed by changing only thecounter electrode, and withstand voltage of TFT and the increase of theconsumption electric power due to the increase of voltage of the gatesignal line drive circuit which is to be a problem at the time when theelectric current supplying line is greatly changed, can be suppressed.Furthermore, the reduction of the electric power consumption isperformed while the enhancement of the reliability is secured by makingthe reverse bias smaller than the forward bias. Moreover, the increaseof the number of electric sources can also be suppressed by making thepotential at the time when the reverse bias is applied be in common withthe potential of the electric source of the source signal line drivecircuit or the gate signal line drive circuit.

1. A light emitting device comprising: a source signal line providedover a substrate; a gate signal line for writing provided over saidsubstrate; a gate signal line for blanking provided over said substrate;an electric current supplying line provided over said substrate; aswitching transistor provided in a pixel and over said substrate; adriving transistor provided in said pixel and over said substrate; ablanking transistor provided in said pixel and over said substrate; apixel electrode provided in said pixel and over said substrate andconnected with one of a source region and a drain region of said drivingtransistor; a counter electrode provided over said substrate; anelectroluminescence element provided in said pixel and comprising alight emitting layer, said light emitting layer provided over saidsubstrate and between said pixel electrode and said counter electrode;and a switch connected with said counter electrode for switching apotential of said counter electrode from one of a first potential and asecond potential to the other, wherein one of a source region and adrain region of said switching transistor is connected with said sourcesignal line, and the other of said source region and said drain regionof said switching transistor is connected with a gate electrode of saiddriving transistor, and a gate electrode of said switching transistor isconnected with said gate signal line for writing, wherein one of asource region and a drain region of said driving transistor is connectedwith said electric current supplying line, and the other of said sourceregion and said drain region of said driving transistor is connectedwith said pixel electrode, wherein a gate electrode of said blankingtransistor is connected with said gate signal line for blanking, and oneof a source region and a drain region of said blanking transistor isconnected with said electric current supplying line, and the other ofsaid source region and said drain region of said blanking transistor isconnected with said gate electrode of said driving transistor, whereinsaid first potential is applied to said counter electrode to apply aforward bias voltage between said pixel electrode and said counterelectrode, and said second potential is applied to said counterelectrode to apply a reverse bias voltage between said pixel electrodeand said counter electrode, and wherein said reverse bias voltage haspolarity inversed with respect to said forward bias voltage.
 2. A lightemitting device according to claim 1 wherein said light emitting deviceis incorporated into one selected from the group consisting of acellular phone, an EL display, a mobile computer and a game machine. 3.A light emitting device according to claim 1 wherein said reverse biasvoltage is smaller than said forward bias voltage.
 4. A light emittingdevice according to claim 1 wherein said switching transistor is anN-channel type TFT, and said blanking transistor is an N-channel typeTFT, and said driving transistor is a P-channel type TFT.
 5. A lightemitting device according to claim 4 wherein a side connected to saiddriving transistor is an anode, and a side connected to said counterelectrode is a cathode.
 6. A light emitting device according to claim 1wherein said pixel electrode comprises a transparent conductive film. 7.A light emitting device according to claim 1 wherein said counterelectrode comprises a transparent film.
 8. A light emitting devicecomprising: a source signal line provided over a substrate; a gatesignal line for writing provided over said substrate; a gate signal linefor blanking provided over said substrate; an electric current supplyingline provided over said substrate; a switching transistor provided in apixel and over said substrate; a driving transistor provided in saidpixel and over said substrate; a blanking transistor provided in saidpixel and over said substrate; a pixel electrode provided in said pixeland over said substrate and connected with one of a source region and adrain region of said driving transistor; a counter electrode providedover said substrate; an electroluminescence element provided in saidpixel and comprising a light emitting layer, said light emitting layerprovided over said substrate and between said pixel electrode and saidcounter electrode; and a switch connected with said counter electrodefor switching a potential of said counter electrode from one of a firstpotential and a second potential to the other, wherein one of a sourceregion and a drain region of said switching transistor is connected withsaid source signal line, and the other of said source region and saiddrain region of said switching transistor is connected with a gateelectrode of said driving transistor, and a gate electrode of saidswitching transistor is connected with said gate signal line forwriting, wherein one of a source region and a drain region of saiddriving transistor is connected with said electric current supplyingline, and the other of said source region and said drain region of saiddriving transistor is connected with said pixel electrode, and wherein agate electrode of said blanking transistor is connected with said gatesignal line for blanking, and one of a source region and a drain regionof said blanking transistor is connected with said electric currentsupplying line, and the other of said source region and said drainregion of said blanking transistor is connected with said gate electrodeof said driving transistor.
 9. A light emitting device according toclaim 8 wherein said light emitting device is incorporated into oneselected from the group consisting of a cellular phone, an EL display, amobile computer and a game machine.
 10. A light emitting deviceaccording to claim 8 wherein said switching transistor is an N-channeltype TFT, and said blanking transistor is an N-channel type TFT, andsaid driving transistor is a P-channel type TFT.
 11. A light emittingdevice according to claim 10 wherein a side connected to said drivingtransistor is an anode, and a side connected to said counter electrodeis a cathode.
 12. A light emitting device according to claim 8 whereinsaid pixel electrode comprises a transparent conductive film.
 13. Alight emitting device according to claim 8 wherein said counterelectrode comprises a transparent film.